The invention relates to a signal-source arrangement comprising
a group of signal sources for generating substantially identical unit signals PA1 correction means for reducing mutual deviations in the unit signals from the signal sources by calibrating the signal sources in conformity with a calibration pattern recurring at a specific period, resulting in unit signals comprising similarly shaped spurious signals recurring at said period, and PA1 combination means for combining a number of unit signals of the available unit signals to form an output signal.
The invention also relates to a digital-to-analog converter comprising such a signal-source arrangement. In the appended claims and the description, "signal source" is to be understood to mean both a voltage source and a current source, so that the corresponding signals should be regarded as voltages and currents respectively.
An arrangement of the type defined in the opening paragraph is known from "A Self Calibration Technique for Monolithic High-Resolution D/A Converters", Digest of Technical Papers, 1989 IEEE International Solid-State Circuits Conference (ISSCC '89), pp. 22-23, February 1989. This known arrangement comprises a group of substantially identical current sources. The current sources each supply a unit current formed by the source-drain current of a MOS transistor. The magnitude of the source-drain current is dictated by a control voltage between the gate and the source of the MOS transistor. Combination means enable the unit currents to be combined in order to form one or more output currents which are multiples of the unit current. This is the case, for example, in a digital-to-analog converter in which the output currents are derived from the unit currents in conformity with a binary-ascending series.
As a result of the spread in transistor parameters, the unit currents supplied by the MOS transistors are never perfectly equal to each other. This imposes a limitation on the accuracy of a digital-to-analog converter comprising such current sources. In the known current-source arrangement the equality of the current sources is improved in that the current sources are recalibrated continually by correction means. By means of a control circuit the control voltage across a capacitor arranged between the gate and source of the MOS transistor is then controlled in such a way that the source-drain current is equal to a reference current. When all the current sources have been calibrated the cycle is repeated. By means of this calibration method it is possible to obtain a group of current sources which are identical to each other with a very high accuracy. However, a consequence of the method is that the control voltage between the gate and the source of the MOS transistors does not remain constant between two consecutive calibration cycles as a result of charge leakage of the capacitor between the gate and source electrodes of the MOS transistors. This gives rise to a ripple on the control voltage so that the unit currents may also exhibit ripple components. Moreover, other spurious signals may arise as a result of calibration. For example, the alternate turn-on and turn-off of each of the current sources by means of the control circuit is attended by switching transients on the gates of the MOS transistors, which give rise to a spurious signal in the drain-source current. Therefore, calibration results in all of the unit currents exhibiting a substantially similar spurious signal having a period corresponding to the duration of one calibration cycle.
When the unit currents are combined to form larger output currents, the spurious signals cumulate. The accumulation of the spurious signals restricts the accuracy of the output currents, which in the case of a digital-to-analog converter results in a reduced accuracy and dynamic range.